Propellent charge



May 8, 1962 L. G. BONNER PROPELLENT CHARGE Filed June 17, 1948 FIG. 4

y/nan 6I Bonner FlC INVENTOR.

AGENT.

United States Patent OA 3,033,117 PROPELLENT CHARGE Lyman G. Bonner,Cumberland, Md., assignor to Hercule Powder Company, Wilmington, Del., acorporation of Delaware` Filed June 17, 1948, Ser. No. 33,582 5 Claims.(Cl. 102-98) This invention relates to propellent charges and moreparticularly to neutral burning propellent charges for rockets, thrustunits and other gasfproducing devices.

In the prior art, rocket propellent granulations have been primarilyconfined to singly perforated cylinders, star perforated grains andoutside restricted end burning grains. The singly perforated cylindricalgrain is undesirable in that the chamber wall of the rocket motor isexposed to the hot combustion gases. lf this grain is restricted on itsouter surface, it becomesrprogressive in burning and gives a highlyfluctuating thrust. The star perforated grain does not expose thechamber wall to the hot combustion gases but does give undesirably lowloading densities, especially in short grains. Because of itsconfiguration, this grain burns very rapidly and is therefore usefulonly for charges of comparatively short burning times. Conversely,` endburning grains restricted on their outer surfaces are useful only whencharges are desired which have comparatively long burning times.Furthermore, to obtain the mass rate of gas discharge essential to evenmoderate thrust levels with end burning grains it is necessary to employgrains of undesirably larger diameter. It is not feasible to hold thegrain diameter low and employ propellants of higher burning rate becausethe faster burning and therefore hotter burning charges have undesirabletemperature ycoeicients.

More recently a charge has been developed which comprises an outsiderestricted, perforated cylindrical grain and an unrestricted solidcylindrical grain loaded in tandem. By proper adjustment of the lengthof each of these grains, the regression in the surface of the solidgrain can be made to compensate exactly for the progression of theperforated grain during all stages of burning. While this charge issubstantially neutral in burning surface and does give a desirable massrate of gaseous discharge, it still does not overcome the difficultiesof low loading density and exposing chamber walls to combustion gases.Furthermore, this charge is necessarily bulky and it is diicult tosupport the solid grain centrally in the chamber during burning and thusprevent a shift in center of gravity. In addition, little control can beexercised `over port area, this fact contributing to the low loadingdensity.

In accordance with the present invention, neutral burning propellentcharges of high loading density may be produced which have a widelatitude in burning time, permit a wide choice of powder web andsurface, give a satisfactory mass rate of gaseous discharge and whichafford complete chamber wall protection during burning.

Generally described, this invention comprises a perforated outerpropellent member, the outer surfaces of which are protected to preventcombustion, and having an inner propellent member disposed Within theperforation of the outer member in spaced relationship to form a`combustion area therebetween. The inner core is proportioned so thatduring burning its regression in burning surface substantiallycompensates for the progression in burning Surface of the outer member.If the ends of both members are protected from combustion, exactneutrality `of burning surface may be obtained in charges of anyIlength. However, if the length of the entire charge is great incomparison with` its cross-sectional area, the ends need not berestricted since the surface lost by longitudinal burning will be anegligible fraction of the total 3,@33Jl7 Patented May 8, 1S62 HCCburning surface. Highest loading densities are obtained when the innermember is the samelength as the outer member and its radius is the sameas the thickness of the outer member.

FIG. l is a cross-sectional view of a propellent charge made accordingto one embodiment of this invention.

FIG. 2 is a diagrammatic View of one end of a charge prepared inaccordance with a further embodiment of the invention.

FIG. 3 is an elevational end view of the trapping assembly employed inFIG. l.

FIG. 4 is a sectional side view of the trapping assembly of FIGS. 1 and3.

FIG. 5 is a sectional side view of an embodiment of a compositeinhibitor plate for the ends of the charge in accordance with theinvention.

FIG. 6 is a fragmentary sectional view of a loaded charge in accordancewith the further embodiment of the invention.

In FIG. l a solid cylindrical propellent core 10 is centrally located ina cylindrically perforated propellent cylinder 11, which is restrictedfrom burning on its outer diameter by a jacket of cellulose acetate 12.The ends of the perforated propellent cylinder 11 are restricted fromburning by annular rings of cellulose acetate 13 while the ends of thecentral core l0 are likewise restricted by centrally orifced annularcellulose acetate discs 14. A cylindrical port l5 is left between theoutside diameter of the core 10 and inside diameter of the perforatedcylinder 11 and is of suicient cross-sectional area to allow properventing of combustion gases. The core is secured in its central positionthroughout burning and the entire charge held in place by a suitabletrapping system 30 described in detail in connection with FIGS. 3 and 4.The entire charge is contained in snug, slidable relationship within acombustion chamber 1 of a rocket-actuated device (not shown). Threadedlyengaged with the chamber 1 is a discharge nozzle 2. The nozzle 2 holdsthe trapping assembly 30 in position when screwed into place followingloadingof the charge. A similar trapping assembly is placed at the otherend of the charge and held in place in similar fashion by the memberattached thereto such as a warhead or a second discharge nozzle if thedevice is to be employed as an optional thrust unit.

In FIG. 2 the ends of the outer cylindrical member 20,

its surface inhibiting material 21, and the core 22 are ber and size ofthe connecting segments as Well as ther thickness of the entire platemay be varied according to the size and weight of the components of thecharge. Furthermore, the inhibitor plate may be made to incorporaterecesses or projections to engage the trapping and/ or supportingsystems of the various rocket motors or other thrust devices.

In FIGS. 3 and 4 is shown a simple trapping means which comprises aflat, outer, annular metallic ring 31 connected to a concentricallydisposed, pointed, metallic stud 32 by four thin radially spacedmetallic spokes 33. VThe pointed stud passes through the orifice of theinhibitor disc 14 and when the nozzle 2 is screwed into place, the pointof the stud 32 is forced slightly into-the end of the solid propellentcore 10. Together with a similar trapping assembly disposed at the otherend of the charge, the core 10 is maintained in its concentric locationwithin the outer propellent member 11. The trapping assemblies arepreferably made from steel or other metal capable of withstanding thetemperature of the combustion gases without distortion. The inhibitordisc 14 need not be orificed if well bonded to the end of core It) sinceit has been found that adequate support may be obtained if the point ofthe stud projects only into the inhibitor disc.

In FIG. 5 is shown a further embodiment of a composite inhibitor platemade from cellulose acetate and disposed in operative relationship witha further embodiment of a trapping system which may be employed inaccordance with the invention. An annular outer ring 50 is connected tothe concentrically disposed disc 51 by means of radially disposedsegments as shown at 24 in FIG. 2. An annular recess 52 is formed at theperiphery of the outer ring 50. The trapping system 53 comprises anouter ring 54 of rectangular cross section. A concentrically disposed,pointed stud 55 having an annular flange 56 is joined to the edge of thering 54 by evenly spaced spokes 57. The ring 54 rests in recess 52 ofthe inhibitor plate, the pointed stud 55 passes through the centralorifice of disc 51, and the flange 56 rests against the disc 51 when, asshown in FIG. l, the discharge nozzle or warhead is screwed into place.As in the trapping assembly of FIGS. l, 3, and 4, the point of the studneed not project into the core of the charge but may simply engage thedisc 51. If the disc 51 is orificed, the stud should lill the orificeand thus prevent the combustion gases from igniting the end of the core1t) behind the inhibitor disc.

In FIG. 6, a charge in accordance with the invention consisting of anouter member 60 and an inner core 61 is disposed in tight, slidable fitwithin `a steel combustion chamber 62. The wall of the combustionchamber acts as the inhibiting means and prevents combustion on theouter surface of the member 60.

Having now described the nature and purpose of the present invention,the following example is given:

Propellent charges were prepared from the following ingredients: v

Percent Nitrocellulose (13.15% N) 58.5 Nitroglycerin 22.5 Ethylcentralite 8.0 Dinitrotoluene 2.5 Triacetin 8.5 Lead Stearate (added)0.5

The charges were designed to give a V200-pound thrust for 4 seconds andhave the following specifications:

Length of perforated cylinder inches 23.50 Length of solid core do 23.50Outer diameter of perforated cylinder do 5.27 Inner diameter ofperforated `cylinder do 2.87 Outer diameter of solid cylindrical core do2.40 Ratio of port area to nozzle throat area 52.5

The outer surfaces of the perforated cylinders of these grains wererestricted longitudinally with a cellulose acetate jacket. To achieveperfect neutrality of burning surface it was necessary to also restrictthe ends of both the outer cylinder and the core but in this particulargrain, highly saisfactory results were obtained by restricting only theends of the solid core with cellulose acetate discs 1/s inch thick.These discs were cemented into place with acetone as the cementingmedium. Satisfactory neutrality of burning was also obtained when theends of yboth the perforated cylinder and solid core were leftunrestricted since in this particular granulation the length is so greatin relation to cross-sectional area that the surface lost from endwiseburning of both perforated cylinder and core is a nearly negligible`fraction of the total surface. In static tests, these charges give aminimum of thrust fluctuation and excellent neutral burning pressuretimeand thrust-time records Showing sharp ends of burning. Propellantcharges of this granulation have been extensively tested in gliderthrust units, which are symmetrical rockets designed to give optionaldirection of thrust, with highly satisfactory results.

It has been found that only very simple trapping systems are necessarywhen charges are prepared in accordance with this invention. This is dueto the fact that because the grain is restricted on its outer surface,the chamber may be fitted closely around the periphery of the outercylinder and is supported by the chamber Wall. Since little or no powdersilvering is encountered with the charges of this invention the primaryfunction of the trapping system is to center the solid core and hold itin place. This can be accomplished by a simple trap at each end of thegrain.

Because the charge is restricted from burning on its outer surface, theilow of hot combustion gases over the inner wall of the chamber tubingis eliminated. This makes possible the use, for chamber construction, ofmaterials such as aluminum alloys which have excellent strength toweight ratios at ordinary temperatures but which lose strength rapidlyat elevated temperatures.

FIt is possible to obtain very high loading densities in chargesprepared in accordance with this invention. For a given powder web andweight, grain length and outside diameter may be adjusted to give theminimum acceptable port area which results in the maximum loadingdensity. Conversely, for a given outside diameter and length, the weband weight may be adjusted to produce the same effect.

The charges of this invention may be fabricated by extrusion or casting.If prepared by extrusion, it is preferred to extrude the outer cylinder.and central core separately. The outer surafce of the perforatedcylinder and the ends of the perforated cylinder and solid core may thenbe restricted with cellulose acetate or other nonflammable material asdesired. Extrusion is preferred in grains of small diameter but whengrains of diameter of six inches or more are desired the extrusionprocess becomes undesirable both because of the massive and expensiveequipment necessary and the hazards involved.

The charges may be cast safely and easily by utilizing the apparatus andprocess disclosed in the copending application of Gordon W. McCurdy,Serial No. 28,218, led May 20, 1948. According to this process, thecasting powder is introduced into a cylindrical cellulose acetatecasting container of proper size, equipped with a hollow cylindricalcore which forms the port of the charge and to which the casting willnot adhere. The casting powder may be tightly packed into the containerby tamping or vibration. Pressure is then exerted on the powder at thetop of the container and a suitable casting liquid is admitted to thebottom of the container through a manifold base assembly. This baseassembly is so constructed that the incoming liquid is evenlydistributed over the cross-sectional area at the base of the containerand any air entrained in the casting liquid system is trapped. After theliquid has risen through and covered the casting powder, the castingliquid system is disconnected and the entire apparatus is stored at asuitable temperature until cured.

After curing, the casting container with its contents is removed fromthe base assembly. The cellulose acetate casting container is preferablyutilized as the restrictive covering for the outside surface of theouter cylinder. However, in case the casting container employed isconstructed of a material which is undesirable as an inhibitingmaterial, the casting may be removed from the casting container aftercuring and restricted with cellulose acetate or other suitable materialas desired. If desired, the ends of both cylinder and core may berestricted by cementing or otherwise bonding annular cellulose acetaterings to the ends of the outer cylinder and cellulose acetate discs tothe ends of the core. However, in order to enable shipment,y storage andloading of the two commay be coated with a thin film of silicone greaseor other suitable lubricant prior to introduction of the casting powder.Unce the `form has been extracted another composite inhibitor plate isthen bonded to the opposite ends. Once the charge is loaded, the radialconnecting segments between the inner and outer portions of theinhibitor plate are consumed by initial combustion and the trapping andsupporting systems then maintain the core in spaced relationship withthe outer propellent member during burning.

Alternatively, the outer cylinder and core may be cast separately, orthe outer cylinder may be cast and the core extruded. The two componentsmay then be assembled and maintained in spaced relationship by use ofthe composite inhibitor plate. However, casting the entire chargesimultaneously is desirable in order to insure t. that the componentspossess substantially identical burning characteristics.

In certain applications, having the hot combustion gases contact thechamber walls at the end of burning may not be disadvantageous. It maynot be necessary, therefore, to inhibit the peripheral surfaces of theouter member of the charge with an independent means. In that ease, thegas-producing device-may be so designed that the chamber wall serves asthe inhibiting means.

The size of the cylindrical port left between the inner member and theouter member depends on the particular granulation and on the particularrocket motor, thrust unit, or gas-producing device into which thechargev is to be loaded. Based on empirical knowledge, the mostsatisfactory results are obtained when 2.5 is taken as the limitingratio of port area to nozzle throat area.

Propellant charges produced in accordance with this invention have manyadvantages. Charges may be designed with a wide choice of web andsurface while maintaining a high loading density. Since the chamberwalls can at all times be protected from hot combustion gases, lighterand less refractory materials can be used in construction of thechamber. Furthermore, the

charges may be made absolutely neutral in burning surface or can be maderegressive or progressive to the degree desired. It is therefore to beunderstood that the example given is presented for illustrative purposesand that the invention is not limited by the charge described thereinbut only by the scope of the appended claims.

What I claim and desire to protect by Letters Patent is:

1. A substantially neutral burning propellant charge comprising acylindrical outer member of combustible explosive composition having acentrally disposed cylindrical perforation longitudinally thereof, meansdisposed about the curved outer surface of said outer member whichprotects said surface from combustion, and a solid cylindrical innermember of combustible explosive composition disposed within theperforation in spaced relationship to form an annular combustion zonebetween the members, the radius of said inner member being equal to thethickness of the outer member.

2. The charge of claim il wherein the surfaces which are protected toprevent combustion are so protected by means associated with the devicecontaining the charge.

3. The charge of claim 1 wherein the surfaces which are protected toprevent combustion are so protected by means suitably bonded tothecharge itself.

4. The charge of claim 1 wherein the end surfaces are protected toprevent combustion by means bonded to the charge, said means being unitsof flame-resistant material comprising an outer annular ring joined to aconcentrically disposed disc and being adapted to maintain the innermemberk in spaced relationship to the outer member prior to combustion.

5. A propellent charge in accordance with claim l wherein the inner andouter propellent members are of equal length.

References Cited in the le of this patent UNITED STATES PATENTS 622,777McGahie Apr. 11, 1899 654,471 Maxim July 24, 1900 1,018,312 GherassimoiFeb. 20, 1912 FOREIGN PATENTS 26,430 Great Britain 1907 502,560 FranceFeb. 24, 1920 516,865 Great Britain Ian. 12, 1940

